In some cellular radio network standards, a common Radio Base Station (RBS) may instantaneously transmit to several User Equipments (UE), using Orthogonal Frequency Division Multiplexing (OFDM) within the available bandwidth, i.e. the bandwidth allocated for the RBS transmission in one cell.
OFDM modulation gives high EPF (Envelope Peak Factor), which reduces the available power from a user equipment. In a Radio Base Station peak reduction methods (clipping) are often used to reduce the ratio between the peak power and the average power of the sent signal. The added peak reduction energy can be distributed to all carriers within the radio bandwidth or located to special reserved tones carrying no data for OFDM. When locating the peak reduction energy to all tones within the bandwidth errors are introduced to the tones carrying information. A measurement of how much error that is introduced is the Error Vector Amplitude (EVM) measurement. Low EVM means small distortion of the message sent and received. The envelope peak reduction methods are often only implemented in the RBS as a mobile User Equipment is hardware limited and can not afford the added digital hardware used for clipping. This means that the analogue power amplifier for the OFDM user equipment in Up-link (UL) has to be designed for higher peak power capability than the base station. This will limit the available transmit power from the UE when also considering the available power from the UE battery. Mobile systems are often UL power limited as the case for OFDM.
Therefore, the use of Localized- or Distributed Single carrier modulations for the UE are proposed in order to reduce the EPF and provide higher user equipment analogue power for transmission in the power amplifier. The SC-FDMA signals are in contrary to OFDM generated from modulation symbols in the time domain and will give less EPF for the same type of modulation used in OFDM.
A commonly used modulation like QPSK gives low EPF and low data transfer availability, while 16-QAM and 64-QAM have higher EPF and higher data transfer. The drawback is that low EPF signal modulation will decrease the available data rate for the transmission
A preferred aspect of OFDM modulation is the availability to insert pilot tones in the transmitted symbol or time signal. The pilot tones are used to calibrate the radio channel and the receiver frequency dependent errors mainly occurring in the receiver filtering. Normally receiver filtering is implemented as SAW-filters (Surface Acoustic Wave Filters) due to the need of good receiver selectivity. Such a receiver may introduce 10-15% EVM mainly emerging from the receiver filtering. But by using the pilot tones spread over the radio channel bandwidth, the frequency response over the channel and the receiver filtering is calibrated and the EVM influence can almost be eliminated leaving the an OFDM receiver. EVM values in the range of 2-3% typical can be achieved emerging mainly from frequency generation phase noise in the used oscillators of the receiver and the transmitter.
The large EPF of OFDM can be reduced by reserving an amount of optimally spread reserved tones for peak reduction. The peak cancelling energy is allocated to the reserved tones which carries no data. In this case the peak reduction does not introduce any errors into the tones carrying data information. FIG. 1 shows the EPF reduction in an OFDM 20 MHz channel using two stage clip application with Gaussian spread pilot tones occupying 6.25% of the available tones of the OFDM modulation. As the clipping functionality only uses the reserved tones, the peak reduction of the OFDM signal EVM is not increased for the tones used for the transmitted message. In the figure the left curve is representing the clipped performance and the right curve is the original OFDM EPF distribution.
By reserving tones for both calibration and clipping in OFDM a certain percentage of the available data rate in the channel is lost. The pilot tones for receiver calibration do not always need to be transmitted and can be transmitted in certain intervals depending on the radio channel changes. Different pilot selections can also be made over a block of sent time signals so the whole block will use the summed pilot tone responses for calibration in the reception.
The EPF reduction by peak clipping is done for reducing the size and DC-power needed for the RF power amplifiers used to provide the OFDM-modulation in the air interface. The pilot tone calibration technique improves the reception of OFDM by providing better C/I (Carrier to Interference ratio) for the received OFDM message.
FIG. 2 illustrates the procedure of generation of localised Single Channel-Frequency Division Multiple Access (SC-FDMA) and distributed SC-FDMA modulations in known art. This is described in 3GPP standardization technical reports: 3GPP TR25.814 V0 3.01.
No-symbols are generated in time domain, wherein No is the used number of carriers allocated to the radio channel. The symbols can be Quadrature Phase-Shift Keying (QPSK), 8 Phase Shift Keying (8-PSK), 16-Quadrature Amplitude Modulation (16QAM) or other higher order linear modulations. A Discrete Fourier Transform (DFT) 110 of size No tones or bins is generated. The No tones are then mapped (sub carrier mapping 120) into a radio channel with tones numbers size greater than N. The size of the radio channel is 2^n tones like 512, 1024 or 2048 etc where the Fast Fourier Transform (FFT) 130 can be executed efficiently. Then a new time function is created by inverse FTT techniques (IFFT). Cyclic prefix and ramp window for the symbol to be transmitted are added 140 and the symbol is transmitted over the radio interface.
The preference for SC-FDMA modulation in the UL in a mobile system is the lower EPF achieved for the modulation. As the message symbols are generated in the time domain EPF figures are like the achievable EPF for the modulation used. π/4-QPSK gives the lowest EPF and thus power consumption and complexity of the user equipment Analogue Radio Frequency (RF)-transmitter. The drawback is the rather high EVM occurring in reception of SC-FDMA, if the receiver frequency response is not calibrated. The equalising techniques must be done in the time domain.
The aim of reducing the EPF of the user equipment modulation will give reduced data rate in the user equipment and there are no methods to use pilot tones within the modulation symbol to improve the reception of SC-FDMA symbol. Methods to use a whole symbol for calibration tones are although available. Training sequences can be put into the time domain which then implies that the reception of SC-FDMA needs advanced equalisers used in other linear modulations used in GSM and WCDMA systems. The availability of such timed domain designed equalisers intended for taking care of the receiver introduced EVM is limited, without introducing very large amount of training sequences in the modulation.
In order to use the fairly simple equalising techniques used for OFDM, calibration tones are required. This is not so efficient when using SC-FDMA modulation. To provide pilot tones for calibration, an entire IFFT time sequence must be used. The known art of generating calibrating tones is as follows: The message for providing calibration tones in SC-FDMA to be sent is No/X long X-times repeated in the time domain. This gives that the time sequence sent will be used only for channel and receiver calibrating without any further information available in this message. Tones for calibration will be at equal intervals of X tones. How often the calibration message will be needed is dependent on the radio channel variation versus time.
The problem with the known art LC- or SC-FDMA modulation methods are:                low order modulation must be used because of EPF demands from the user equipment reduces the achievable data rate,        the EPF improvement by peak reduction is complex and introduces EVM on the transmitted signal,        equalising in the time domain is needed        availability to provide receiver calibration and use frequency domain equalisers as for OFDM is limited and reduces the data rate more than for an OFDM modulation.        